Solid electrolyte type fuel cell and air electrode current collector for used therein

ABSTRACT

A solid electrolyte type fuel cell which incorporates a metal separator comprising a base material of a metal other than silver or a silver alloy which is plated with silver or a silver alloy. The fuel cell can achieve improved efficiency for electricity generation with no increase of the resistance of the metal separator, even when it is operated at a low temperature.

TECHNICAL FIELD

The present invention relates to a solid electrolyte type fuel cellincluding an electric power generation cell constituted by anelectrolyte layer sandwiched between a fuel electrode layer and an airelectrode layer, and having high output density even when it is operatedat a low temperature, and particularly to an air electrode currentcollector used for such a low temperature operating solid electrolytetype fuel cell.

BACKGROUND OF THE INVENTION

In general, since a solid electrolyte type fuel cell can use hydrogengas, natural gas, methanol, coal gas or the like as a fuel, it canpromote the substitution of alternative energy for oil in electric powergeneration, and further, since waste heat can be used, it has attractedattention from the viewpoint of resource savings and an environmentalproblem.

This solid electrolyte type fuel cell has a laminated structure as shownin an exploded perspective view of FIG. 1 and a sectional schematic viewof FIG. 2. That is, a solid electrolyte type fuel cell 10 includes anelectric power generation cell 14 made up of a solid electrolyte layer11, a fuel electrode layer 12 and an air electrode layer 13, which aredisposed at both sides of this solid electrolyte layer 11, a fuelelectrode current collector 16 disposed to be in close contact with thefuel electrode layer 12, an air electrode current collector 18 disposedto be in close contact with the air electrode layer 13, and metalseparators 17 constructed so that a fuel gas can be supplied to the fuelelectrode layer 12 and an oxidizing agent gas containing oxygen can besupplied to the air electrode layer 13. Reference numerals 20 and 21 inFIG. 2 denote grooves which become a fuel passage and an air passage,respectively.

The conventional solid electrolyte type fuel cell is operated at a hightemperature of 1000° C., so that chemical energy of fuel can berelatively efficiently converted into electric energy, however, in orderto operate the solid electrolyte type fuel cell at 1000° C., materialsused for component parts of the solid electrolyte type fuel cell arerestricted especially to materials superior in heat resistance. Forexample, as a structure material of the separator or the like, it hasbeen necessary to use dense ceramics such as lanthanum chromite(LaCrO₃). Further, an attached apparatus (for example, a preheatingapparatus of fuel gas, or the like) for operating the solid electrolytetype fuel cell is also required to be made of a material resistant tohigh temperature, and because of the operation at a high temperature,the consumption of the material becomes quick, the use life becomesshort, and it is inevitable that the cost becomes high. Thus, in recentyears, a solid electrolyte type fuel cell which can be efficientlyoperated at a temperature lower than 1000° C. and in which metalmaterial can be used for peripheral members, has been developed.

In such a low temperature operating solid electrolyte type fuel cell,lanthanum gallate oxide, Sc-added zirconium, Y-added zirconium, ceriabase oxide or the like is used for a solid electrolyte layer. By usingthese materials, the operation temperature can be lowered to about 700°C., and a metal material can be used for a peripheral member such as aseparator. As a metal separator material, stainless steel, nickel baseheat resistant alloy, cobalt base alloy or the like is used.

Besides, an air electrode current collector material is one of aplurality of important members influencing the power generationperformance of a fuel cell, and mesh-shaped platinum is conventionallyused as the air electrode current collector material.

However, the surface of the conventional metal separator material iscovered with a chromium oxide film under conditions of, for example,700° C. in the air, and the chromium oxide has conductivity at a hightemperature, and has such a property that when temperature is lowered,its electric resistance increases. Accordingly, in the case where theoperation is performed at a low temperature of about 700° C., it has adefect that the electric resistance is too large to be used as aseparator material. Thus, even in the case where the fuel cell isoperated at a low temperature while metal material is used for theseparator, a material having a smaller electric resistance has beenrequired.

Besides, in the case where a platinum mesh is used as an air electrodecurrent collector, platinum is expensive since it is a noble metal, andfor reduction in cost, a high performance air electrode currentcollector material substituting for platinum has been desired.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrode currentcollector used in a solid electrolyte type fuel cell, which isinexpensive as compared with a conventional air electrode currentcollector made of a platinum mesh, and can achieve excellent electricpower generation efficiency even in a case where it is operated at a lowtemperature.

Further, an object of the invention is to provide a relativelyinexpensively solid electrolyte type fuel cell in which electricresistance of a metal separator is not increased even in a case where itis operated at a low temperature, and electric power generationefficiency can be improved.

That is, a first aspect of the invention is an air electrode currentcollector characterized by comprising a silver porous body.

The second aspect of the invention is an air electrode current collectorused for a solid electrolyte type fuel cell, characterized by comprisinga silver porous body having a surface on which an oxide film is formed.

The third aspect of the invention is an air electrode current collectorused for a solid electrolyte type fuel cell, characterized by comprisinga dispersion strengthened silver porous body in which an oxide isdispersed in a silver base metal.

The fourth aspect of the invention is an air electrode current collectorused for a solid electrolyte type fuel cell, characterized by comprisinga silver alloy porous body having a melting point of 600° C. or higher.

The fifth aspect of the invention is, in the fourth aspect of theinvention, an air electrode current collector in which the silver alloyhaving the melting point of 600° C. or higher is a silver alloycontaining not more than 40% by mass of one of or at least two of Cu,Zn, Cd, Ni, Sn, Au, Pt, Pd, Ir and Rh in total and the remainder of Agand an inevitable impurity.

The sixth aspect of the invention is, in the fourth aspect of theinvention, an air electrode current collector in which the silver alloyhaving the melting point of 600° C. or higher the silver alloy havingthe melting point of 600° C. or higher is a dispersion strengthenedsilver porous body in which an oxide is dispersed in a silver alloy basemetal containing not more than 40% by mass of one of or at least two ofCu, Zn, Cd, Ni, Sn, Au, Pt, Pd, Ir and Rh in total and the remainder ofAg and an inevitable impurity.

The seventh aspect of the invention is an air electrode currentcollector used for a solid electrolyte type fuel cell, characterized bycomprising a porous body of metal or alloy having high temperaturestrength more superior than silver, in which a Ni plating under layer isformed on at least a side of the porous body in contact with an airelectrode, and a silver plating is formed thereon.

The eighth aspect of the invention is, in the seventh aspect of theinvention, an air electrode current collector in which the metal or thealloy having the high temperature strength more superior than the silveris Ni or Ni base alloy, Fe or Fe alloy, or Co or Co alloy.

The ninth aspect of the invention is, in any of the first to eighthaspects of the invention, an air electrode current collector in whichthe porous body is a sponge metal porous body having a three-dimensionalskeletal structure.

The tenth aspect of the invention is, in any of the first to ninthaspects of the invention, an air electrode current collector in whichthe porous body is reinforced by a mesh metal body.

The eleventh aspect of the invention is, in the tenth aspect of theinvention, an air electrode current collector in which the mesh metalbody is made of silver or silver alloy, or is made of a metal matrixwhich is other than silver or silver alloy and is coated with silver orsilver alloy.

The twelfth aspect of the invention is, in the eleventh aspect of theinvention, an air electrode current collector in which the mesh metalbody is the metal matrix other than the silver or the silver alloy, themetal matrix is plated with nickel and is plated with silver while thenickel plating is used as an under layer.

The thirteenth aspect of the invention is an air electrode currentcollector used for a solid electrolyte type fuel cell, characterized bycomprising a silver felt made of a silver fiber.

The fourteenth aspect of the invention is an air electrode currentcollector used for a solid electrolyte type fuel cell, characterized bycomprising a silver mesh made of a silver thin wire.

The fifteenth aspect of the invention is an air electrode currentcollector used for a solid electrolyte type fuel cell, characterized bycomprising a silver plated felt made of a silver plated fiber in which asurface of a metal fiber made of metal or alloy having high temperaturestrength more superior than silver is plated with silver.

The sixteenth aspect of the invention is an air electrode currentcollector used for a solid electrolyte type fuel cell, characterized bycomprising a silver plated mesh obtained by giving silver plating to ametal mesh made of a metal thin wire of metal or alloy having hightemperature strength more superior than silver.

The seventeenth aspect of the invention is, in the fifteenth orsixteenth aspects of the invention, an air electrode current collectorin which the metal or the alloy having the high temperature strengthmore superior than the silver is Ni or Ni base alloy, Fe or Fe basealloy, or Co or Co base alloy.

The eighteenth aspect of the invention is a solid electrolyte type fuelcell comprising an air electrode current collector of any one of thefirst to seventeenth aspects of the invention.

According to the air electrode current collector of the first toseventeenth aspects of the invention, as compared with the conventionalair electrode current collector made of the platinum mesh, since the lowcost material can be used, the manufacture cost can be reduced. Besides,according to the solid electrolyte type fuel cell comprising the airelectrode current collector of the first to seventeenth aspects of theinvention, as compared with the conventional solid electrolyte type fuelcell comprising the air electrode current collector made of the platinummesh, the electric power generation efficiency can be improved by afactor of 1.6 or more, and excellent electric power generationcharacteristics can be exhibited even in a case where the operation isperformed at a temperature lowered to 900° C. or lower.

Further, the nineteenth aspect of the invention is, as shown in FIGS. 1and 2, a solid electrolyte type fuel cell comprising an electric powergeneration cell 14 made up of a solid electrolyte layer 11, a fuelelectrode layer 12 and an air electrode layer 13, which are disposed atboth sides of this solid electrolyte layer 11, a fuel electrode currentcollector 16 disposed to be in close contact with the fuel electrodelayer, an air electrode current collector 18 disposed to be in closecontact with the air electrode layer, and metal separators 17constructed so that a fuel gas can be supplied to the fuel electrodelayer and an oxidizing agent gas containing oxygen can be supplied tothe air electrode layer, wherein the solid electrolyte type fuel cell ischaracterized in that the metal separators 17 are plated with one ofsilver and silver alloy. The metal separators are plated with one ofsilver and silver alloy, so that the electric resistances of therespective metal separators 17 can be remarkably decreased for a longtime.

The twentieth aspect of the invention is, in the nineteenth aspect ofthe invention, a fuel cell in which the metal separators 17 are made ofstainless steel, nickel base heat resistant alloy or cobalt base alloy.The stainless steel, the nickel base heat resistant alloy, or the cobaltbase alloy is used for the metal separators, so that excellent heatresistance is exhibited.

The twenty-first aspect of the invention is, in the twentieth aspect ofthe invention, a fuel cell in which the stainless steel is ferritestainless steel. Since the ferrite stainless steel is excellent inadhesion to silver, it is preferable as a metal matrix.

The twenty-second aspect of the invention is, in any one of thenineteenth to twenty-first aspects of the invention, a fuel cell inwhich the metal separators 17 are plated with nickel, and are platedwith one of silver and silver alloy while the nickel plating is used asan under layer. The under plating with nickel is performed, so that highadhesion between the metal separator and the silver or sliver alloyplating can be obtained.

The twenty-third aspect of the invention is, in the nineteenth aspect ofthe invention, a fuel cell in which the air electrode current collector18 is a porous body made of silver or silver alloy, or a porous body inwhich a porous body of a metal other than silver or silver alloy iscoated with silver. Silver has properties that it is reduced even in ahigh temperature oxidizing atmosphere of 200° C. or higher, a solidmetal phase is a stable phase, oxygen is slightly dissolved therein, andoxygen easily diffuses in the inside. On the other hand, theconventionally used platinum hardly dissolves oxygen. Thus, theperformance is improved by using silver as the material of the airelectrode current collector.

The twenty-fourth aspect of the invention of, in the invention of claim23, a fuel cell in which the air electrode current collector 18 is themetal matrix other than the silver or the silver alloy, in which themetal matrix is plated with nickel, and is plated with silver while thenickel plating is used as an under layer. The under plating with nickelis performed, so that high adhesion between the metal matrix and silvercan be obtained.

The twenty-fifth aspect of the invention is, in the twenty-third aspectof the invention, a fuel cell in which the porous body is a sponge metalporous body having a three-dimensional skeletal structure.

The twenty-sixth aspect of the invention is, in the twenty-third ortwenty-fifth aspects of the invention, as shown in FIG. 3, a fuel cellin which the porous body 18 a is reinforced by a mesh metal body 18 b.The porous body 18 a is brittle and there is also a case where it iseasily crushed, and in this case, the shape of the air electrode currentcollector can be kept more firmly by performing the reinforcement withthe mesh metal body 18 b.

The twenty-seventh aspect of the invention is, in the twenty-sixthaspect of the invention, a fuel cell in which the mesh metal body 18 bis made of silver or silver alloy, or is made of a metal matrix which isother than silver or silver alloy and is coated with silver or silveralloy. The cell performance can be stabilized by making the mesh metalbody 18 b out of the same material as the air electrode currentcollector.

The twenty-eighth aspect of the invention is, in the twenty-seventhaspect of the invention, a fuel cell in which the mesh metal body 18 bis the metal matrix other than the silver or the silver alloy, the metalmatrix is plated with nickel, and is plated with silver while the nickelplating is used as an under layer. The high adhesion between the metalmatrix and the silver can be obtained by performing the under platingwith nickel.

The twenty-ninth aspect of the invention is, in any of the nineteenth totwenty-eighth aspects of the invention, a fuel cell in which the solidelectrolyte layer 11 is a conductor selected from a group consisting oflanthanum gallate solid oxide, Sc-stabilized zirconium, Y-stabilizedzirconium, and ceria base oxide. When these conductors are used as thesolid electrolyte layer 11, the fuel cell with the operation temperatureof lower than 950° C. can be easily realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view showing a basic structure of asolid electrolyte type fuel cell.

FIG. 2 is a sectional schematic view showing a laminated structure ofthe solid electrolyte type fuel cell more plainly.

FIG. 3 is a schematic view showing an embodiment of an air electrodecurrent collector used for a solid electrolyte type fuel cell of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIGS. 1 and 2, a solid electrolyte type fuel cell 10includes an electric power generation cell 14 made up of a solidelectrolyte layer 11, a fuel electrode layer 12 and an air electrodelayer 13, which are disposed at both sides of this solid electrolytelayer 11, a fuel electrode current collector 16, an air electrodecurrent collector 18, and metal separators 17 constructed so that a fuelgas can be supplied to the fuel electrode layer 12 and an oxidizingagent gas containing oxygen can be supplied to the air electrode layer13, and is constructed so as to operate at a temperature lower than 950°C.

The solid electrolyte layer 11 is formed of lanthanum gallate solidoxide, Sc-stabilized zirconium, Y-stabilized zirconium, or ceria baseoxide.

The fuel electrode layer 12 is made of metal such as Ni, or cermet suchas Ni—YSZ (Ni doped Y₂O₃ stabilized ZrO₂). Alternatively, it is formedof a mixture of Ni and a compound expressed by a general formula (1):Ce_(1−m)D_(m)O₂. Where, in the general formula (1), D denotes one kindof or not less than two kinds of elements selected from a groupconsisting of Sm, Gd, Y and Ca, and m denotes an atomic ratio of the Delement and is preferably set to be within a range of 0.05 to 0.4,preferably, 1 to 0.3.

The air electrode layer 13 is formed of an oxide ion conductor expressedby a general formula (2): Ln1_(1−x)Ln2_(x)E_(1−y)Co_(y)O_(3+d). Where,in the general formula (2), Ln1 is one element of or both elements of Laand Sm, Ln2 is one element or both elements of Ba, Ca and Sr, and E isone element or both elements of Fe and Cu. Besides, x denotes an atomicratio of Ln2 and is set to be within a range of more than 0.5 and lessthan 1.0. Besides, y denotes an atomic ratio of the Co element and isset to be within a range of more than 0 and not larger than 1.0,preferably, from 0.5 to 1.0. Besides, d is set within a range of from−0.5 to 0.5.

The electric power generation cell 14 is fabricated in such a mannerthat the fuel electrode layer 12 is formed on one side of the solidelectrolyte layer 11, and the air electrode layer 13 is further formedon the other side of the solid electrolyte layer 11, and they are bakedat 1000° C.

The metal separator 17 is formed of a metal other than silver or silveralloy. This metal separator is plated with one of silver and silveralloy. By plating the metal separator with silver or silver alloy, theelectric resistance can be remarkably decreased for a long time. Themetal separator 17 is plated with nickel, and is plated with silver orsilver alloy while this nickel plating is used as an under layer, sothat the adhesion between the metal separator 17 and the silver or thesilver alloy is improved.

The material of the metal separator includes stainless steel, nickelbase heat resistant alloy, and cobalt base alloy. The stainless steelincludes SUS 430 (18Cr—Fe), SUS 310S (20Ni-25Cr—Fe), SUS 316(18Cr-12Ni-2.5Mo—Fe) and the like, the nickel base heat resistant alloyincludes Inconel 600 (15.5Cr-7Fe—Ni), Inconel 718 (19Cr-3Mo-19Fe—Ni),Haynes alloy 214 (16Cr-2Fe-4.5Al—Ni), Haynes alloy 230(16Cr-2Mo-14W—Ni), Hastelloy C-22 (22Cr-13Mo-3W-4Fe—Ni) and the like,the cobalt base alloy includes ULTMET (26Cr-5Mo-2W-3Fe-9Ni—Co), Haynesalloy 188 (22Cr-14.5W—Co) and the like. As the stainless steel, ferritestainless steel is preferable since it has excellent adhesion to silver.A method of plating the metal separator with silver or silver alloyincludes electroplating. This electroplating method is a surfacetreatment method for electrochemically depositing (electrode position)metal on the surface of metal or nonmetal.

The metal separator 17 includes an air inlet 17 a and a fuel gas inlet17 c at its side portions, and includes an air blowoff port 17 b forguiding the air introduced in the air inlet 17 a to the air electrodelayer 13, and a fuel gas blowoff port 17 d for guiding the fuel gasintroduced in the fuel gas inlet 17 c to the fuel electrode layer 12.Further, as shown in FIG. 2, it includes a groove 20 as a fuel passageat a side of the metal separator in contact with the fuel electrodecurrent collector 16 and a groove 21 as an air passage at a side incontact with the air electrode current collector 18.

The fuel electrode current collector 16 is a porous body made ofplatinum, nickel or silver.

The air electrode current collector 18 is a porous body 18 a made ofsilver or silver alloy, or a porous body in which a porous body of metalother than silver or silver alloy is coated with silver. This porousbody 18 a is made of a skeletal portion (skeleton) and pores as shown ina partial enlarged view of FIG. 3, and is a sponge metal porous bodyhaving a three-dimensional structure. It is preferable that its porosityis within a range of 60 to 97%.

In a temperature range of not lower than 200° C. and lower than 950° C.,silver is reduced even in an oxidizing atmosphere, and a solid metalphase becomes a stable phase. Accordingly, in the porous body having thesurface made of silver, an oxide film is not formed in the temperaturerange of not lower than 200° C. and lower than 950° C., and it is anexcellent conductor. However, when the solid electrolyte type fuel cellincorporating the air electrode current collector made of the silverporous body is operated at a temperature of lower than 950° C., althoughan oxide film is not produced on the surface of the air electrodecurrent collector made of the silver porous body, since silver dissolvesoxygen at a high temperature, it starts to melt at about 950° C. Thus,it is desirable that the operation temperature of the solid electrolytetype fuel cell incorporating the air electrode current collector of theporous body made of the silver or the silver alloy or the porous body inwhich the metal matrix other than the silver or the silver alloy iscoated with silver, is lower than 950° C. It is preferably lower than930° C.

In the case where the air electrode current collector 18 is the porousbody in which the porous body other than the silver or the silver alloyis coated with silver, the metal matrix other than the silver or thesilver alloy is plated with nickel, and is plated with silver while thisnickel plating is used as the under layer, so that the high adhesionbetween the metal matrix and the silver can be obtained, which ispreferable.

Besides, as shown in FIG. 3, the porous body 18 a may be reinforced by amesh metal body 18 b. This mesh metal body 18 b is silver or silveralloy, or a metal body in which a metal matrix other than silver orsilver alloy is coated with silver or silver alloy. In the case wherethe porous body 18 a and the mesh metal body 18 b are such that themetal matrix other than the silver or the silver alloy is coated withsilver, the metal matrix includes nickel, stainless, nickel base alloy,cobalt base alloy and the like. The metal matrix is plated with nickel,and is plated with silver while this nickel plating is used as the underlayer, so that the high adhesion between the metal matrix and the silvercan be obtained, which is preferable. The opening of the mesh metal body18 b is within a range of 0.5 to 1000 μm.

It is conceivable that the reason why the electric power generationperformance of the solid electrolyte type fuel cell incorporating theair electrode current collector of the porous body containing silver isimproved at a low temperature is as follows: In general, in the airelectrode layer, oxygen in the air receives an electron by the airelectrode current collector, and an oxygen ion (O⁻²) is generated,however, in the case where silver containing a very small amount ofoxygen is made the air electrode current collector, the very smallamount of oxygen contained in the air electrode current collector has afunction to promote generation of the oxygen ion at the surface of thecurrent collector, and the oxygen ion can be quickly moved from thesurface of the current collector; by a rise in exchange current densitybetween the air electrode current collector and the electrode, themovement of the oxygen ion becomes faster; and dissociation (O₂→2O) ofoxygen and ionization (O+2e→O⁻²) are also promoted by oxygen dissolvedin the air electrode current collector made of the porous bodycontaining silver.

The operation of the solid electrolyte type fuel cell constructed asshown in FIGS. 1 and 2 will be described. When the fuel gas (H₂, CO,etc.) is introduced into the fuel gas inlet 17 c, it passes through thepores in the fuel electrode current collector 16 and is quickly suppliedto the fuel electrode layer 12. On the other hand, when air isintroduced into the air inlet 17 a, it passes through the pores in theair electrode current collector 18 and is quickly supplied to the airelectrode layer 13. The oxygen supplied to the air electrode layer 13passes through the pores in the air electrode layer 13, reaches thevicinity of an interface with the solid electrolyte layer 11, receivesan electron from the air electrode layer 13 at this portion, and isionized into an oxide ion (O²⁻). This oxide ion diffuses and moves inthe solid electrolyte layer 11 toward the fuel electrode layer 12, andwhen it reaches the vicinity of an interface with the fuel electrodelayer 12, it reacts with the fuel gas at this portion to produce areaction product (H₂O, CO₂, etc.), and releases an electron into thefuel electrode layer 12. This electron is extracted by the fuelelectrode current collector 16, so that a current is generated, andelectric power is obtained.

The description has been made such that the foregoing air electrodecurrent collector made of the porous body of silver or silver alloy iscombined with the metal separator plated with one of silver and silveralloy and is incorporated in the solid electrolyte type fuel cell.

However, the air electrode current collector made of the porous body ofsilver or silver alloy is not necessarily used in combination with themetal separator plated with one of silver and silver alloy, and as longas the solid electrolyte type fuel cell has the laminated structure asshown in FIGS. 1 and 2, for example, it can be used in combination withthe separator made of ceramics, for example, lanthanum chromite. The airelectrode current collector described below is described as the airelectrode current collector widely used for the solid electrolyte typefuel cell having the laminated structure shown in FIGS. 1 and 2.

That is, the air electrode current collector of the invention may be anoxide adhesion porous body in which an oxide film is formed on thesurface of a silver porous body to increase the mechanical strength, inaddition to the porous body of silver or silver alloy. The oxide to beadhered to the surface of the oxide adhesion porous body includesaluminum oxide, titanium oxide, silicon oxide and the like.

The air electrode current collector of the solid electrolyte type fuelcell has a role to function as a flow passage in which air as theoxidizing agent gas flows. Accordingly, it is further preferable thatthe silver porous body used as the air electrode current collector ofthe solid electrolyte type fuel cell is a dispersion strengthened silverporous body in which an oxide is dispersed in the silver matrix toimprove the mechanical strength.

The oxide contained in the dispersion strengthened silver in which theoxide is dispersed in the silver matrix, specifically includes tinoxide, indium oxide, lanthanum oxide, copper oxide, chromium oxide,titanium oxide, aluminum oxide, iron oxide, nickel oxide, vanadiumoxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide,and the like, and the tin oxide is most preferable. It is preferablethat the content of the oxide contained in this dispersion strengthenedsilver is 3 to 50 vol. %, and the reason is that when it is less than 3vol. %, strengthening as the air electrode current collector of thesolid electrolyte type fuel cell is insufficient, and when it exceeds 50vol. %, the function as the air electrode current collector is lowered,and sufficient output density can not be obtained. Then, it is furtherpreferable that this dispersion strengthened silver porous body isconstructed to have the outermost surface substantially made of silverand the inside made of dispersion strengthened silver.

In the case where the porous body of silver alloy is used as the airelectrode current collector, the silver alloy with a melting point of600° C. or higher (preferably, 800° C. or higher) is used. Although anyalloys may be used as long as they are silver alloys having the meltingpoint of 600° C. or higher, among these silver alloys, a silver alloycontaining not more than 40% by mass of one of or at least two of Cu,Zn, Cd, Ni, Sn, Au, Pt, Pd, Ir and Rh in total, and the remainder of Agand an inevitable impurity can be preferably used.

The reason why the content of one of or at least two of Cu, Zn, Cd, Ni,Sn, Au, Pt, Pd, Ir and Rh is made 40% by mass or less in total is thatwhen it exceeds 40% by mass, the catalytic function of Ag is lowered,which is not preferable.

It is further preferable that the silver alloy porous body having themelting point of 600° C. or higher is a dispersion strengthened silverporous body in which an oxide is dispersed in a silver alloy matrixcontaining not more than 40% by mass of one of or at least two of Cu,Zn, Cd, Ni, Sn, Au, Pt, Pd, Ir and Rh in total and the remainder of Agand an inevitable impurity.

Further, the air electrode current collector used for the solidelectrolyte type fuel cell of the invention may be constituted by aporous body which is made of a porous body of metal or alloy having hightemperature strength more superior than silver, in which a silverplating layer is formed on at least a side of the porous body in contactwith the air electrode. In order to form the silver plating layer, a Niplating layer is generally formed as an under layer, and the silverplating layer is formed on this Ni plating under layer. A plating methodfor forming this Ni plating under layer and the silver plating layer isnot particularly limited, and they may be formed by any plating method.

It is preferable that the metal or the alloy having the high temperaturestrength more superior than the silver is Ni or Ni base alloy, Fe or Febase alloy, or Co or Co alloy, and specifically, the Ni or Ni base alloyincludes pure Ni, Inconel 600, Hastelloy C-22, Haynes alloy 214 and thelike, the Fe or Fe base alloy includes pure Fe, carbon steel, stainlesssteel, esit steel and the like, and the Co or Co alloy includes Haynesalloy 188, ULTET and the like.

It is sufficient if the porosity of the porous body constituting the airelectrode current collector of the invention and containing silver is 60to 97%. It is preferable that a minute pore does not exist in theskeleton, and in case the minute pore exists in the skeleton, it isnecessary to suppress it to less than 10% of the total. When theporosity of the skeleton becomes 10% or more, the strength as the airelectrode current collector is lowered, which is not desirable.

Further, the air electrode current collector used for the solidelectrolyte type fuel cell of the invention may be constituted by asilver felt made of a silver fiber or a silver mesh made of a silverthin wire.

In the air electrode current collector constituted by the silver felt orthe silver mesh, when the silver felt or the silver mesh is exposed to ahigh temperature for a long time, since silver is low in hightemperature strength, the silver felt and the silver mesh are shrunk andsintered to decrease the void ratio, and the function as the airelectrode current collector of the solid electrolyte type fuel cell canbecome low.

Research has been conducted to obtain an air electrode current collectorconstituted by a silver felt or a silver mesh in which even if the airelectrode current collector is exposed to a high temperature for a longtime, the decrease of a void ratio due to shrinkage is small, andaccordingly, the function as the air electrode current collector is notlowered for a long time. As a result, it has been found that in an airelectrode current collector constituted by a silver plated felt made ofa silver plated fiber in which a metal fiber of metal or alloy havinghigh temperature strength more superior than silver is plated withsilver, or an air electrode current collector constituted by a silverplated mesh obtained by giving silver plating to a metal mesh made of ametal thin wire of metal or alloy having high temperature strength moresuperior than silver, since the metal fiber and the metal thin wire madeof the metal or the alloy having the high temperature strength moresuperior than the silver keep the skeleton at a high temperature, ascompared with the air electrode current collector constituted by thesilver felt made of the silver fiber and the air electrode currentcollector constituted by the silver mesh made of the silver thin wire,even when it is used at a high temperature for a long time, the voidratio of the air electrode current collector is hardly decreased.

It is preferable that the metal or the alloy having the high temperaturestrength more superior than the silver is Ni or Ni base alloy, Fe or Febase alloy, or Co or Co base alloy.

Hereinafter, examples of the invention, together with comparativeexamples, will be described.

EXAMPLES 1 TO 3

First, a pure silver atomized powder having a mean particle diameter of2 μm was prepared. This pure silver atomized power is a powder obtainedby dissolving pure silver by a normal melting furnace and atomizing theobtained pure silver melt. Further, n-hexane as an organic solvent,dodecylbenzene sodium sulphonate (hereinafter referred to as DBS) as asurface active agent, hydroxypropyl methylcellulose (hereinafterreferred to as HPMC) as a water soluble resin binder, glycerin as aplasticizer, and distilled water as water were respectively prepared.

Next, after the pure silver atomized powder and the HPMC (water solubleresin binder) were inserted in a strong shearing type kneading machineand were kneaded for 30 minutes, 50% by mass of all distilled waterrequired to be added was added and kneading was performed. Further, 50%by mass of the remaining distilled water, and other additives of then-hexane (organic solvent), the DBS (surface active agent) and theglycerine (plasticizer) were added and kneading was performed for 3hours, so that mixed slurry containing 50.0% by mass of pure silveratomized powder, 1.5% by mass of n-hexane, 5.0% by mass of HPMC, 2.0% bymass of DBS, and 3.0% by mass of glycerine was prepared. The remainingconstituent is distilled water.

Next, a compact having a thickness of about 1 mm was fabricated fromthis mixed slurry by a doctor blade method, and this compact was foamed,degreased and sintered under conditions shown in Table 1 below, so thata pure silver porous plate having a thickness of about 0.7 mm wasprepared.

TABLE 1 Foaming Degreasing Sintering condition condition conditionAtmosphere — In air In air Moisture 90% — — Temperature 35° C. 450° C.910° C. Holding time 10 min. 60 min. 120 min.

This pure silver porous plate was cut to have a predetermined size, anda pure silver porous body having a three-dimensional skeletal structurewith a porosity of 92 to 97% was produced. As a reinforcing member ofthe pure silver porous body, a mesh metal body made of an expand metalmade of silver was prepared. The pure silver porous body and the meshmetal body were stacked and were subjected to skin pass rolling to formone body, so that the air electrode current collector as shown in FIG. 3was produced.

The solid electrolyte layer was formed of lanthanum gallate solid oxide.The lanthanum gallate solid oxide was produced by the following method.As base powders, respective powders of La₂O₃, SrCO₃, Ga₂O₃, MgO and CoOare prepared, these base powders are respectively weighed to formLa_(0.8)Sr_(0.2)Ga_(0.8)Mg_(0.15)CO_(0.05)O₃, the respective powders aremixed, and this mixture is preliminarily fired at 1100° C. The obtaintemporarily fired body is pulverized, a normal binder, a solvent and thelike are added, and they are pulverized by a ball mill to prepareslurry, and this slurry is shaped into a green sheet by a doctor blademethod. The shaped green sheet is sufficiently dried in the air, and iscut to have a predetermined size, and this is sintered at 1450° C., sothat the lanthanum gallate solid oxide is obtained. Here, the doctorblade method is one of methods for molding into sheet shape, and is amethod in which the thickness of a slip placed on and transferred by acarrier such as a carrier film or an endless belt, the thickness of asheet is precisely controlled by adjusting an interval between a knifeedge called a doctor blade and the carrier.

A nickel porous body was used for the fuel electrode current collector.The solid electrolyte layer was sandwiched between the fuel electrodelayer and the air electrode layer to constitute a single cell of anelectric power generation cell. Next, as metal separator materials,SUS430 (example 1), Inconel 600 alloy (example 2) and ULTMET alloy(example 3) were respectively prepared. The surfaces of the metalseparator materials were plated with silver by an electric platingmethod to form a plating of a thickness of 2 to 5 μm and the metalseparators were formed. Two single cells were stacked to form atwo-stage cell stack, and this two-stage cell stack was sandwichedbetween the metal separators to obtain the fuel cell.

COMPARATIVE EXAMPLES 1 TO 3

Metal separators identical to the examples 1 to 3 except that silverplating was not given to the metal separators of the examples 1 to 3,and platinum porous bodies of 200 mesh were used for air electrodecurrent collectors, were used and fuel cells were prepared similarly tothe examples 1 to 3.

EXAMPLES 4 TO 6

Metal separators identical to the examples 1 to 3 except that the solidelectrolyte layer was formed of Sc-stabilized zirconia, were used, andfuel cells were prepared similarly to the examples 1 to 3.

Sc-stabilized zirconia was produced by the following method. Sc₂O₃ andZrOCl₂ are made starting materials, a predetermined amount of Sc₂O₃ isadded as a nitric acid solution to a monoclinic ZrO₂ sol obtained byhydrolysis of a ZrOCl₂ solution, urea is added, temperature is kept at90° C., they are made to be uniformly deposited, and this deposit istemporarily fired at 600° C. This temporarily fired body is fired at1400° C. for one hour, so that Sc-stabilized zirconia is obtained.

COMPARATIVE EXAMPLES 4 TO 6

Metal separators identical to the examples 4 to 6 except that silverplating was not given to the metal separators of the examples 4 to 6,and platinum porous bodies of 200 mesh were used for air electrodecurrent collectors, were used and fuel cells were prepared similarly tothe examples 4 to 6.

EXAMPLES 7 TO 9

Metal separators identical to the examples 1 to 3 except that solidelectrolyte layers were formed of y-stabilized zirconia using a 8% Y₂O₃doped ZrO₂ powder, were used and fuel cells were prepared similarly tothe examples 1 to 3.

Y-stabilized zirconia was produced by the following method. Y₂O₃ andZrOCl₂ are made starting materials, a predetermined amount of Y₂O₃ isadded as a nitric acid solution to a monoclinic ZrO₂ sol obtained byhydrolysis of a ZrOCl₂ solution, urea is added, a temperature is kept at90° C., they are made to be uniformly deposited, and this deposit istemporarily fired at 600° C. This temporarily fired body is fired at1400° C. for one hour, so that Y-stabilized zirconia is obtained.

COMPARATIVE EXAMPLES 7 TO 9

Metal separators identical to the examples 7 to 9 except that silverplating was not given to the metal separators of the examples 7 to 9,and platinum porous bodies of 200 mesh were used for air electrodecurrent collectors, were used and fuel cells were fabricated similarlyto the examples 7 to 9.

EXAMPLES 10 TO 12

Metal separators identical to the examples 1 to 3 except that solidelectrolyte layers were formed of gadolina doped ceria base oxide usinga Ce_(0.9)Gd_(0.1)O_(1.95) powder, were used and fuel cells werefabricated similarly to the examples 1 to 3.

The gadolina doped ceria base oxide was produced by the followingmethod. CeO₂ and Gd₂O₃ are mixed to obtain a composition ofCe_(0.9)Gd_(0.1)O_(1.95), and are temporarily fired at 1250° C. for 20hours. This temporarily fired body is fired at 1600° C. for 30 hours, sothat the gadolina doped ceria base oxide is obtained.

COMPARATIVE EXAMPLES 10 TO 12

Metal separators identical to the examples 10 to 12 except that silverplating was not given to the metal separators of the examples 10 to 12,and platinum porous bodies of 200 mesh were used for air electrodecurrent collectors, were used and fuel cells were prepared similarly tothe examples 10 to 12.

<Comparative Estimation>

The fuel cells of the examples 1 to 12 and the comparative examples 1 to12 were operated to generate electricity at 700° C. for 500 hours whilea hydrogen gas as a fuel gas was supplied at 3 cc/cm²/minute, and air asan oxidizing agent gas was supplied at 15 cc/cm²/minute, and theperformance evaluation of electric power generation output of therespective fuel cells after 500 hours had passed was performed. Here,the electric power generation performance was evaluated by a mean valueof outputs per single cell obtained by adjusting a potential differencebetween the fuel electrode current collector and the air electrodecurrent collector to 0.7 V and making measurements. Table 2 showselectric power generation performances of the fuel cells of the examples1 to 12 and the comparative examples 1 to 12, respectively.

TABLE 2 Mean Value of Metal Silver Current Collector Solid ElectrolyteSingle Cell Separator Plating Air Electrode Fuel Electrode Layer Output[mW/cm²] Ex. 1 SUS430 With Silver Porous Nickel Porous Lanthanum Gallate465 Ex. 2 Inconel 600 With Body Body Solid Oxide 470 Ex. 3 ULTMET With +472 Ex. 4 SUS430 With Reinforcing Sc- Stabilized 160 Ex. 5 Inconel 600With Member Zirconia 219 Ex. 6 ULTMET With 221 Ex. 7 SUS430 With Y-Stabilized 126 Ex. 8 Inconel 600 With Zirconia 129 Ex. 9 ULTMET With 131Ex. 10 SUS430 With Gadolina Doped 231 Ex. 11 Inconel 600 With Ceria BaseOxide 240 Ex. 12 ULTMET With 241 Comp. Ex. 1 SUS430 Without PlatinumLanthanum Gallate 298 Comp. Ex.2 Inconel 600 Without Porous Body SolidOxide 295 Comp. Ex. 3 ULTMET Without 290 Comp. Ex. 4 SUS430 Without Sc-Stabilized 106 Comp. Ex. 5 Inconel 600 Without Zirconia 145 Comp. Ex. 6ULTMET Without 142 Comp. Ex. 7 SUS430 Without Y- Stabilized  81 Comp.Ex. 8 Inconel 600 Without Zirconia  84 Comp. Ex. 9 ULTMET Without  84Comp. Ex. 10 SUS430 Without Gadolina Doped 153 Comp. Ex. 11 Inconel 600Without Ceria Base Oxide 162 Comp. Ex. 12 ULTMET Without 165

As is apparent from Table 2, as compared with the comparative examples 1to 12 in which the metal separators were not plated with silver and theplatinum porous bodies were used for the air electrode currentcollectors, in the examples 1 to 12 in which the same electrolytematerials were used, the metal separators were plated with silver, andthe silver porous bodies were used for the air electrode currentcollectors, the output mean values per unit cell exceeded.

EXAMPLE 13

A pure silver porous body plate having a thickness of 1.5 mm wasproduced similarly to the production method shown in the example 1, thispure silver porous body plate was cut, and an air electrode currentcollector made of the pure silver porous body having a porosity shown inTable 3 was prepared.

Further, similarly to the production method shown in the example 1, alanthanum gallate solid oxide sintered body having a thickness of 110 μmwas produced, and this was made a solid electrolyte layer. A mixturepowder of NiO and (Ce_(0.8)Sm_(0.2))O₂ in which they were mixed so thata volume ratio of Ni to (Ce_(0.8)Sm_(0.2))O₂ was 6:4, was fired to oneside of this solid electrolyte layer at 1100° C. to form a fuelelectrode layer, and further, (Sm_(0.5)Sr_(0.5))CoO₃ was fired to theother side of the solid electrolyte layer at 1000° C. to form an airelectrode layer, so that a cell was fabricated.

Further, after a lanthanum chromite powder was subjected to isostaticpressing to form a plate, a groove was formed by machining, and next,sintering at 1450° C. was performed, so that a separator having thegroove a tone side was fabricated. Besides, a Ni felt was prepared as afuel electrode current collector.

The Ni felt as the fuel electrode current collector was stacked at thefuel electrode side of the cell fabricated in this way, the airelectrode current collector made of the pure silver porous body wasstacked at the air electrode side of the cell, and further, theseparators were stacked on the fuel electrode current collector and theair electrode current collector, so that a solid electrolyte type fuelcell 1 having the structure shown in FIG. 2 was prepared.

CONVENTIONAL EXAMPLE 1

Further, for comparison, an air electrode current collector made of aplatinum mesh was prepared, and a conventional solid electrolyte typefuel cell 1 was prepared similarly to the example 1 except that insteadof the air electrode current collector of the invention made of the puresilver porous body of the example 13, the air electrode currentcollector made of the platinum mesh was incorporated.

While the solid electrolyte type fuel cell 1 of the invention obtainedin this way and the conventional solid electrolyte type fuel cell 1 werekept at 700° C., a dry hydrogen gas was made to flow as a fuel gas, airwas made to flow as an oxidizing agent gas, current density at 0.7 V wasmeasured with respect to the solid electrolyte type fuel cell 1 of theinvention and the conventional solid electrolyte type fuel cell 1, andthe results were shown in Table 3.

TABLE 3 Air Electrode Current Collector Current Density Type CompositionPorosity (%) (mA/cm²) at 0.7 V Solid Electrolyte Type Fuel 1 Pure Silver92 595 Cell of the Invention Conventional Solid 1 Platinum Mesh 360Electrolyte Type Fuel Cell

From the results shown in Table 3, it is understood that the solidelectrolyte type fuel cell 1 of the invention in which the air electrodecurrent collector made of the pure silver porous body is incorporated isgreatly improved in the current density at 0.7 V as compared with theconventional solid electrolyte type fuel cell 1 in which the airelectrode current collector made of the platinum mesh fabricated in theconventional example 1 is incorporated.

EXAMPLE 14

As oxide powders, SnO₂ powder having a mean particle diameter of 0.5 μm,In₂O₃ powder having a mean particle diameter of 0.5 μm, La₂O₃ powderhaving a mean particle diameter of 0.5 μm, and Fe₂O₃ powder having amean particle diameter of 0.5 μm, all of which were commerciallyavailable, were prepared.

The SnO₂ powder, the In₂O₃ powder, the La₂O₃ powder, or the Fe₂O₃ powderwas mixed to the pure silver atomized powder prepared in the example 13,and they were pulverized and mixed by a ball mill for 100 hours toperform mechanical alloying, so that a silver-oxide dispersionstrengthened alloy powder in which oxide was dispersed in the inside wasprepared. The obtained silver-oxide dispersion strengthened alloy powderwas used, and was shaped and sintered under the same condition as theexample 13, so that the air electrode current collectors made of thedispersion strengthened silver porous bodies having the compositions andporosities shown in Table 4 were fabricated. By stacking the airelectrode current collectors made of these dispersion strengthenedsilver porous bodies to air electrode sides of cells, solid electrolytetype fuel cells 2 to 5 of the invention having the structure shown inFIG. 2 were fabricated similarly to the example 13. With respect to thesolid electrolyte type fuel cells 2 to 5 of the invention, currentdensity at 0.7 V was measured and the results were shown in Table 4.

TABLE 4 Air Current Collector Made of Dispersion Current StrengthenedSilver Density Composition (vol. %) (mA/cm²) at Type Oxide Ag Porosity(%) 0.7 V Solid Electrolyte 2 SnO₂:12 Remainder 91 578 Type Fuel Cell of3 In₂O₃:10 Remainder 92 581 the Invention 4 La₂O₃:11 Remainder 91 585 5Fe₂O₃:10 Remainder 95 555

From the results shown in FIG. 4, it is understood that the solidelectrolyte type fuel cells 2 to 5 of the invention in each of which theair electrode current collector made of the dispersion strengthenedsilver porous body is incorporated are greatly improved in the currentdensity at 0.7 V as compared with the conventional solid electrolytetype fuel cell 1 of Table 3 fabricated in the conventional example 1.

EXAMPLE 15

The air electrode current collector made of the pure silver porous bodyfabricated in the example 13 was made a base body, an Al₂O₃ film havinga thickness of 5 μm was formed by vacuum evaporation on the surface ofthis base body to fabricate an oxide adhesion air electrode currentcollector with increased mechanical strength, and a solid electrolytetype fuel cell 6 of the invention was fabricated in which the oxideadhesion air electrode current collector with the increased mechanicalstrength was incorporated. As a result of measurement of current densityat 0.7 V with respect to the solid electrolyte type fuel cell 6 of theinvention, the measured current density is 583 mA/cm², and it isunderstood that this value is greatly improved as compared with theconventional solid electrolyte type fuel cell 1 of Table 3 fabricated inthe conventional example 1.

EXAMPLE 16

As silver alloy powders, silver alloy atomize powders each having a meanparticle diameter of 1.5 μm and having a composition shown in Table 5were prepared. These silver alloy atomize powders were used, and shapingand sintering were performed under the same condition as the example 13,so that air electrode current collectors made of the silver alloy porousbodies having compositions and porosities shown in Table 5 werefabricated. By stacking the air electrode current collectors made ofthese dispersion strengthened silver porous bodies to air electrodesides of cells, solid electrolyte type fuel cells 7 to 20 of theinvention having the structure shown in FIG. 2 were prepared similarlyto the example 13. With respect to the solid electrolyte type fuel cells7 to 20 of the invention, current density at 0.7 V was measured and theresults were shown in Table 5.

TABLE 5 Air Current Collector Made of Silver Alloy Porous BodyComposition (% by mass) Porosity Current Density Type Cu Zn Cd Ni Sn AuPt Pd Ir Rh Ag (%) (mA/cm²) at 0.7 V Solid 7 18 — — — — — — — — —Remainder 93 551 Electrolyte 8 25 10 — — — — — — — — Remainder 93 555Type 9 18 10 6 — — — — — — — Remainder 93 590 Fuel 10 — — 26  2 — — — —— — Remainder 90 560 Cell 11 16 11 — — 3 — — — — — Remainder 96 556 ofthe 12 25 — — 1 — — — — — — Remainder 92 571 Invention 13 — 10 — — 1 — —— — — Remainder 94 562 14 — — 3 — 4 — — — — — Remainder 91 585 15 — — 72 — — — — — — Remainder 91 570 16 — — — — — 5 — — — — Remainder 93 58817 — — — — — — 3 — — — Remainder 91 587 18 — — — — — — — 4 — — Remainder94 585 19 — — — — — — — — 2 — Remainder 94 590 20 — — — — — — — — — 1Remainder 93 593

From the results shown in Table 5, it is understood that the solidelectrolyte type fuel cells 7 to 20 of the invention each incorporatingthe air electrode current collector made of the silver alloy porous bodycontaining not more than 40% by mass of one of or at least two of Cu,Zn, Cd, Ni, Sn, Au, Pt, Pd, Ir and Rh in total, and the remainder of Agand an inevitable impurity are greatly improved in the current densityat 0.7 V as compared with the conventional solid electrolyte type fuelcell 1 of Table 3 fabricated in the conventional example 1.

EXAMPLE 17

Air electrode current collectors made of dispersion strengthened silveralloy porous bodies having compositions and porosities in which oxideswere uniformly dispersed in the silver alloy porous body matrixes usedfor the solid electrolyte type fuel cells 7 to 20 of the invention ofthe example 16 were stacked to air electrode sides of cells, so thatsolid electrolyte type fuel cells 21 to 34 of the invention having thestructure shown in FIG. 2 were fabricated similarly to the example 13.With respect to the solid electrolyte type fuel cells 21 to 34 of theinvention, current density at 0.7 V was measured and the results wereshown in Table 6.

TABLE 6 Air Current Collector Made of Dispersion Strengthened SilverAlloy Porous Body Current Density Composition (vol. %) Porosity (mA/cm²)at Type Oxide Silver Alloy (%) 0.7 V Solid 21 SiO₂:8 Silver Alloy Usedin Solid Electrolyte Type Fuel Cell 7 of 93 563 Electrolyte theInvention: Remainder Type 22 TiO₂:13 Silver Alloy Used in SolidElectrolyte Type Fuel Cell 8 of 93 532 Fuel the Invention: RemainderCell 23 Fe₂O₃:7 Silver Alloy Used in Solid Electrolyte Type Fuel Cell 9of 95 542 of the the Invention: Remainder Invention 24 NiO:15 SilverAlloy Used in Solid Electrolyte Type Fuel Cell 10 92 540 of theInvention: Remainder 25 MgO:32 Silver Alloy Used in Solid ElectrolyteType Fuel Cell 11 94 505 of the Invention: Remainder 26 CaO:21 SilverAlloy Used in Solid Electrolyte Type Fuel Cell 12 93 538 of theInvention: Remainder 27 SiO₂:8 Silver Alloy Used in Solid ElectrolyteType Fuel Cell 13 93 550 of the Invention: Remainder 28 TiO₂:13 SilverAlloy Used in Solid Electrolyte Type Fuel Cell 14 91 561 of theInvention: Remainder 29 Fe₂O₃:7 Silver Alloy Used in Solid ElectrolyteType Fuel Cell 15 93 559 of the Invention: Remainder 30 NiO:15 SilverAlloy Used in Solid Electrolyte Type Fuel Cell 16 94 571 of theInvention: Remainder 31 MgO:32 Silver Alloy Used in Solid ElectrolyteType Fuel Cell 17 94 526 of the Invention: Remainder 32 CaO:21 SilverAlloy Used in Solid Electrolyte Type Fuel Cell 18 92 543 of theInvention: Remainder 33 SiO₂:8 Silver Alloy Used in Solid ElectrolyteType Fuel Cell 19 93 574 of the Invention: Remainder 34 TiO₂:13 SilverAlloy Used in Solid Electrolyte Type Fuel Cell 20 91 572 of theInvention: Remainder

From the results shown in Table 6, it is understood that the solidelectrolyte type fuel cells 21 to 34 of the invention incorporating theair electrode current collectors made of the dispersion strengthenedsilver alloy porous bodies are greatly improved in the current densityat 0.7 V as compared with the conventional solid electrolyte type fuelcell 1 of Table 3 fabricated in the conventional example 1.

EXAMPLE 18

As alloy powders having high temperature strength more superior thansilver, respective atomized powders of SUS430 (composition, Cr: 17% iscontained, and the remainder is Fe and an inevitable impurity), SUS304(composition, Ni: 9.3% and Cr: 18.4% are contained, and the remainder isFe and an inevitable impurity), Ni-10% Cr alloy, INCONEL 600 (Cr: 15.5%and Fe: 7% are contained, and the remainder is Ni and an inevitableimpurity), Haynes alloy 188 (Ni: 22%, Cr: 22% W: 14.5%, and Fe: 1.5% arecontained, and the remainder is Co and an inevitable impurity), eachhaving a mean particle diameter of 2.1 μm and shown in Table 7, wereprepared, and these alloy atomized powders were used to perform shaping,and sintering was performed in vacuum at temperature shown in Table 7,so that alloy porous bodies having porosities shown in Table 7 werefabricated. After Ni plating under layers having thicknesses shown inTable 7 were formed at one sides of the alloy porous bodies, Ag platinglayers were formed, so that air electrode current collectors werefabricated. By using these air electrode current collectors, similarlyto the example 13, solid electrolyte type fuel cells 35 to 39 of theinvention having the structure shown in FIG. 2 were fabricated, and withrespect to the solid electrolyte type fuel cells 35 to 39 of theinvention, current density at 0.7 V was measured, and the results wereshown in Table 7.

TABLE 7 Air Current Collector Made of Alloy Porous Kind of Alloy BodyHaving High Temperature Strength Powder Having High More Superior ThanSilver Current Temperature Sintering Thickness of Ni Thickness ofDensity Strength More Temperature Porosity Plating Under Ag Plating(mA/cm²) Type Superior Than Silver (° C.) (%) Layer (μm) Layer (μm) at0.7 V Solid 35 SUS430 1100 94 3 10 600 Electrolyte 36 SUS304 1100 91 3 5554 Type Fuel 37 Ni-10% Cr 1100 92 3 5 574 Cell of the 38 INCONEL 6001100 94 3 5 560 Invention 39 Haynes Alloy 188 1100 94 3 5 571

From the results shown in FIG. 7, it is understood that the solidelectrolyte type fuel cells 35 to 39 of the invention incorporating theair electrode current collectors in which Ni plating and Ag plating aregiven to at least the one sides of the porous bodies of the alloyshaving the high temperature strength more superior than the silver aregreatly improved in the current density at 0.7 V as compared with theconventional solid electrolyte type fuel cell 1 of Table 3 fabricated inthe conventional example 1.

EXAMPLE 19

A pure silver fiber made of pure silver and having a mean thickness of30 μm and a mean length of 2 mm, and a pure silver powder having a meanparticle diameter of 2 μm were prepared and combined to have 90% by massof pure silver fiber and 10% by mass of pure silver power, and they weremixed to fabricate a mixture powder of the pure silver fiber and thepure silver powder. This mixture powder was filled into a metal mold andwas slightly subjected to press molding, and then, it was fired at 910°C. for 10 minutes, so that a pure silver felt having a void ratio of 80%and a thickness of 0.7 mm was fabricated, and this pure silver felt wasused to fabricate an air electrode current collector made of the puresilver felt.

Similarly to the example 13 except that the air electrode currentcollector made of the pure silver felt fabricated in the above was usedinstead of the pure silver porous body in the example 13, a solidelectrolyte type fuel cell 40 of the invention having the laminatedstructure shown in FIG. 2 was fabricated using a solid electrolytelayer, a fuel electrode layer, an air electrode layer, a fuel electrodecurrent collector, and separators.

While the solid electrolyte type fuel cell 40 of the invention obtainedin this way was kept at a temperature of 700° C., a dry hydrogen gas asa fuel as was made to flow, and air as an oxidizing agent gas was madeto flow, and with respect to the solid electrolyte type fuel cell 40 ofthe invention, current density at 0.7 V was measured and the result wasshown in Table 8.

Incidentally, for comparison, a measurement value of the current densityat 0.7 V with respect to the conventional solid electrolyte type fuelcell 1 fabricated in the conventional example 1 is also shown in Table8.

EXAMPLE 20

A pure silver thin wire made of pure silver and having a mean thicknessof 20 μm was prepared. A pure silver mesh was fabricated by using thispure silver thin wire, and an air electrode current collector made ofthis pure silver mesh was fabricated. By stacking the air electrodecurrent collector made of this pure silver mesh was stacked at an airelectrode side of a cell, a solid electrolyte type fuel cell 41 of theinvention having the structure shown in FIG. 2 was fabricated similarlyto the example 19. With respect to the solid electrolyte type fuel cell41 of the invention, current density at 0.7 V was measured and theresult was shown in Table 8.

EXAMPLE 21

A Ni fiber having a mean thickness of 20 μm and a mean length of 3 mmwas prepared. The surface of this Ni fiber was plated with pure silverso that a pure silver plated fiber was fabricated, and this pure silverplated fiber was filled into a metal mold, and after press molding wasslightly performed, it was fired at 900° C. for 10 minutes, so that apure silver plated felt having a void ratio of 82% and a thickness 0.7mm was fabricated, and this pure silver plated felt was used tofabricate an air electrode current collector made of the pure silverplated felt. The air electrode current collector made of this puresilver plated felt was stacked at an air electrode side of a cell, sothat a solid electrolyte type fuel cell 42 of the invention having thestructure shown in FIG. 2 was fabricated similarly to the example 19,and with respect to the solid electrolyte type fuel cell 42 of theinvention, current density at 0.7 V was measured and the result wasshown in Table 8.

EXAMPLE 22

A pure Ni thin wire made of pure Ni and having a mean thickness of 30 μmwas prepared. The surface of a Ni mesh fabricated by this Ni thin wirewas plated with pure silver to fabricate a pure silver plated mesh, andan air electrode current collector made of this pure silver plated meshwas fabricated. By stacking the air electrode current collector made ofthis pure silver plated mesh at an air electrode side of a cell, a solidelectrolyte type fuel cell 43 of the invention having the structureshown in FIG. 2 was fabricated similarly to the example 19, and withrespect to the solid electrolyte type fuel cell 43 of the invention,current density at 0.7 V was measured and the result was shown in Table8.

TABLE 8 Structure of Air Electrode Current Collector Incorporated inSolid Current Density Type Electrolyte Type Fuel Cell (mA/cm²) at 0.7 VSolid Electrolyte Type Fuel Pure Silver Felt Made of Pure Silver 603Cell 40 of the Invention Fiber Solid Electrolyte Type Fuel Pure SilverMesh Made of Pure Silver 591 Cell 41 of the Invention Thin Wire SolidElectrolyte Type Fuel Pure Silver Plated Felt Made of Pure 576 Cell 42of the Invention Silver Plated Fiber in Which Surface of Ni Fiber IsPlated with Pure Silver Solid Electrolyte Type Fuel Pure Silver PlatedMesh in Which 560 Cell 43 of the Invention Surface of Ni Mesh Made of NiThin Wire Is Plated with Pure Silver Conventional Solid Platinum Mesh360 Electrolyte Type Fuel Cell 1

From the results shown in Table 8, it is understood that all of thesolid electrolyte type fuel cell 40 of the invention incorporating theair electrode current collector made of the pure silver felt, the solidelectrolyte type fuel cell 41 of the invention incorporating the airelectrode current collector made of the pure silver mesh, the solidelectrolyte type fuel cell 42 of the invention incorporating the airelectrode current collector made of the pure silver plated felt, and thesolid electrolyte type fuel cell 43 of the invention incorporating theair electrode current collector made of the pure silver plated mesh aregreatly improved in current density at 0.7 V as compared with theconventional solid electrolyte type fuel cell 1 incorporating the airelectrode current collector fabricated in the conventional example 1 andmade of the platinum mesh.

INDUSTRIAL APPLICABILITY

As described above, according to the solid electrolyte type fuel cell ofthe invention, since the metal separator in which the metal matrix otherthan the silver or the silver alloy is plated with one of silver andsilver alloy is used, the electric resistance can be remarkablydecreased for a longtime. Besides, as the air electrode currentcollector, the porous body made of silver or silver alloy, or the porousbody in which the metal matrix other than the silver or the silver alloyis coated with silver or silver alloy is used, so that oxygen isdissolved in the inside of silver, and oxygen easily diffuses theinside. As a result, even in the case where it is operated at a lowtemperature, the electric resistance of the metal separator is notincreased, and the electric power generation efficiency can be improved.

Further, since the air electrode current collector of the invention isconstituted by the silver porous body, the silver porous body in whichthe oxide film is formed on the surface of the silver porous body, thedispersion strengthened silver porous body, the silver alloy porous bodyhaving the melting point of 600° C. or higher, the porous body in whichNi plating or Ag plating is given to at least one side of the porousbody of the alloy having high temperature strength more superior thansilver, the silver felt, the silver mesh, the silver plated felt, or thesilver plated mesh, the solid electrolyte type fuel cell incorporatingthe air electrode current collector can improve the electric powergeneration efficiency by a factor of 1.6 or more as compared with theconventional solid electrolyte type fuel cell incorporating the airelectrode current collector made of the platinum mesh. As a result,superior electric power generation characteristics are obtained evenwhen the operation is performed at a temperature lowered to 900° C. orlower, and since the operation can be performed at a low temperature,the use life can be prolonged, and further, since the low cost materialcan be used, the manufacture cost can be reduced, and great contributionto the development of the fuel cell industry is obtained.

1. An air electrode current collector used for a solid electrolyte typefuel cell, the air electrode current collector comprising a silverporous body, wherein the silver porous body is a sponge metal porousbody having a three-dimensional skeletal structure, wherein a porosityof the three-dimensional skeletal structure accounts for less than 10%of a total porosity of the sponge metal porous body, wherein the porousbody is reinforced by a mesh metal body wherein the mesh metal body ismade of silver or silver alloy, or a metal matrix other than silver orsilver alloy and is coated with silver or silver alloy, wherein the meshmetal body is the metal matrix other than the silver or the silveralloy, and wherein the metal matrix is plated with nickel, and is platedwith silver while the nickel plating is used as an under layer.